1. Field of the Invention
This invention relates to multi-layered films, their preparation, and their uses in thermal printing. More particularly, this invention relates to films comprising a substrate and a vapor-deposited colorant layer; to a method of thermal image printing utilizing a donor sheet comprising a substrate, a vapor-deposited colorant layer and a controlled release adhesive layer, and a matching receptor sheet; to a method of imaging; and to a coating process.
2. Description of the Related Art
The technology of thermal pigment transfer systems can generally be divided into two fields, mass transfer and dye sublimation transfer. Thermal imaging technology has been progressing rapidly in the last couple of years, especially in the areas of thermal dye transfer.
The term mass transfer is used to refer to systems in which both the color pigment and its binder are transferred from a donor sheet to a receptor sheet (or intermediate carrier sheet). Because of the relatively large size of the transferred material, a particle comprising both color pigment and binder, color gradation, that is, half-tone image tone is difficult to achieve. Furthermore, in the case of thermal dye transfer, where only dye molecules are transferred through the boundary, extended gradation cannot be achieved. However, dye transfer images generally exhibit more limited aging stability than do color pigments. Additionally, the high energy requirements of 6-10 joules/square centimeters (J/cm.sup.2) in order to achieve thermal dye transfer has been problematic.
While the capabilities of thermal mass transfer printing equipment have improved, the progress of dot growth printing beyond 16 gray levels/pixel has been slow. There is no commercially available matching media that has the resolution capabilities to match the capabilities of printer hardware. Additionally, heat drag problems associated with prolonged printing of printer heating elements can cause uncontrollable dot growth. The low gray level capability of available media, coupled with the difficulty of heat drag control reduces the utility of dot-growth thermal mass transfer technology in graphic arts applications.
Various attempts have been made to eliminate or reduce the limitations described herein above. In the mass transfer area, improvements lie primarily in the design and thermal control of the print head.
This approach was described by S. Maruno of Matsushita Elec. Inc. Co., Ltd. in a paper presented to the August (1986) Society of Photographic Science & Engineering (SPSE) Conference on Non-impact Printing Technologies. "Thermo-convergent ink transfer printing" (TCIP) is described as a system in which the shape of the heating elements of the print head are optimized and the energy pulses to the head are controlled so that continuous tone reproduction is improved when wax-color pigment donor sheets are used.
The donor sheet, itself, has been a subject of improvement work. Japanese Kokai No. 59-224394 discloses the use of two incompatible binders in which the dye is dissolved. This results in the mass transfer of relatively small particles of color pigment. Combining this donor sheet with good print head control has been known to result in a low level of color gradation.
The use of one resin and color pigment in the donor sheet and a different resin in the receptor sheet has been described in a paper by Tagushi et al. of Matsushita given at the SPSE Conference (August, 1986). The modulated thermal signal in the print head causes changes in the "melt, compatibility, adhesion and transfer between the two resins," thereby producing a continually graduated print.
Other examples of improved mass thermal transfer of wax/color pigment systems include: (a) donor sheets incorporating conductive/resistive layer pairs in their constructions and described in U.S. Pat. Nos. 4,470,714 and 4,588,315; and (b) donor sheets containing exothermic materials to amplify the energy provided by the print head and described in U.S. Pat. Nos. 4,491,432 and 4,549,824.
Media using colored dyes and color pigments are used in a wide variety of imaging processes and graphic arts applications. Various technologies, such as color photography, diazonium salt coupling, lithographic and relief painting, dye-bleach color photocopying and photosensitive imaging systems may use dyes or color pigments to form an observable color image. Examples of some of these types of technologies may be found for example in, U.S. Pat. Nos. 3,136,637, 3,671,236, 4,307,182, 4,262,087, 4,230,789, 4,212,936, and 4,336,323. In these systems, the dye or color pigment is present in a carrier medium such as a solvent or a polymeric binder. In the transfer of dyes by sublimation, it has generally been only the final image that consists of essentially pure dye on a receptor sheet. Each of these various imaging technologies has its various complexity, consistency, image quality, speed, stability and expense.
U.S. Pat. No. 4,268,541 describes a method that deposits organic protective layers onto vapor-deposited metal layers. Amongst the organic materials deposited are Rhodamine B and phthalocyanine, a dye and a color pigment. These materials are not described as actively involved in any imaging process.
U.S. Pat. No. 4,271,256 shows image transfer processes using vapor-deposited organic materials, including dyes, where the transfer is made by stripping the image off a substrate with an adhesive film. The reference also discloses the use of dyes under a vapor-coated metal layer to enhance radiation absorption, but does not use a photoresist with the article.
U.S. Pat. No. 3,822,122 describes irradiation of a dye layer (which may have been vapor-deposited) to oxidize or otherwise decolorize the dye and leave an image which can then be transferred to a receptor surface.
U.S. Pat. No. 3,811,884 discloses an image transfer process wherein a layer of organic coloring material is irradiated to color, discolor or fade the material so that the remaining dye image can be transferred by heating.
U.S. Pat. No. 4,587,198 discloses a process for generating a color image comprising exposing a radiation sensitive layer over a vapor-deposited dye or color pigment layer and vaporizing the dye or color pigment to selectively transmit the dye or color pigment through the exposed layer.
U.S. Pat. No. 4,599,298 discloses a radiation sensitive article comprising a substrate, a vapor-deposited dye or color pigment layer capable of providing an optical density of at least 0.3 to a 10 nm band of the EM spectrum between 280 and 900 nm and a vapor-deposited graded metal/-metal oxide or metal sulfide layer. U.S. Pat. No. 4,657,840 discloses a process for producing the article of U.S. Pat. No. 4,599,298.
U.S. Pat. No. 4,705,739 discloses several graphic arts constructions similar to those disclosed in U.S. Pat. Nos. 4,587,198, 4,599,298, and 4,657,840. The constructions disclosed contain an overlaying photosensitive resist layer that must be exposed and developed to obtain an image.
Microstructural and physical properties of vapor-deposited films can depend on deposition conditions, such conditions include (1) substrate temperature, (2) deposition rate, which is a function of the evaporation source temperature, source-to-substrate distance (d.sub.ss), substrate temperature, (3) deposition angles, (4) characteristics of the substrate, and (5) chamber pressure, see for example, Debe and Poirier, Effect of Gravity on Copper Phthalocyanine Thin Films III: Microstructure Comparisons of Copper Phthalocyanine Thin Films Grown in Microgravity and Unit Gravity, Thin Solid Films, 186(1990) 327-347. Thin layers of colorants materials, including CuPc, vapor-deposited at critical substrate temperature generally tend to be smooth and densely packed, and thin layers of vapor-deposited CuPc by physical transport mechanism have been known to show a columnar structure. However, columnar orientation of the vapor-deposited colorant depends on the incident CuPc beam direction during deposition, see Zurong et al., Kexue Tongbao, vol. 29, pg. 280 (1984), which discloses deposition of a colorant layer on a stationary substrate.
In addition to the problems involved in producing low transfer energy, high resolution, and color images, it is essential to utilize a neutral black "color" donor sheet. The neutral black "color" donor sheet should exhibit properties comparable to those of the colorant donor sheets. Conventional carbon black dispersion coating generally can not deliver high resolution. Carbon black vapor coating is generally not considered because of the high melting point (.about.3700.degree. C.) of carbon.
Although most or all of these attempts have been successful to some extent, none has given the desired combination of low transfer energy, high resolution, and full color, continuous half tone images of excellent image color stability, using yellow, magenta, cyan and black (YMCK).